Double substrate reflective spatial light modulator with self-limiting micro-mechanical elements

ABSTRACT

A spatial light modulator includes an upper optically transmissive substrate held above a lower substrate containing addressing circuitry. One or more electrostatically deflectable elements are suspended by hinges from the upper substrate. In operation, individual mirrors are selectively deflected and serve to spatially modulate light that is incident to, and then reflected back through, the upper substrate. Motion stops may be attached to the reflective deflectable elements so that the mirror does not snap to the bottom substrate. Instead, the motion stop rests against the upper substrate thus limiting the deflection angle of the reflective deflectable elements.

CROSS-REFERENCE TO RELATED CASES

[0001] This application is a divisional of U.S. patent application Ser.No. 10/043,703 to Huibers filed on Jan. 9, 2002, which is a continuationof U.S. patent application Ser. No. 09/624,591 to Huibers filed Jul. 24,2000 (now U.S. Pat. No. 6,356,378), which is a continuation of U.S.patent application Ser. No. 09/437,586 to Huibers filed Nov. 9, 1999(now U.S. Pat. No. 6,172,797), which is a continuation of U.S. patentapplication Ser. No. 09/160,361 to Huibers filed Sep. 24, 1998 (now U.S.Pat. No. 6,046,840), which is a continuation-in-part of U.S. patentapplication Ser. No. 08/665,380 filed on Jun. 18, 1996 (now U.S. Pat.No. 5,835,256), which claims priority from a U.S. provisional patentapplication Serial No. 60/000,322 filed on Jun. 19, 1995.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates to spatial light modulators, and,more particularly, to spatial light modulators with electronicallyaddressable deflectable elements attached to an optically transmissivesubstrate.

[0004] 2. Description of Prior Art

[0005] Spatial light modulators (SLMs) are transducers that modulate anincident beam of light in a spatial pattern that corresponds to anoptical or electrical input. The incident light beam may be modulated inphase, intensity, polarization, or direction. This modulation may beaccomplished through the use of a variety of materials exhibitingmagneto-optic, electro-optic, or elastic properties. SLMs have manyapplications, including display systems, optical information processing,optical data storage, and printing.

[0006] A common technology for an SLM cell is to use a liquid crystalmaterial sandwiched between two electrodes, at least one of theelectrodes being transparent. By applying a voltage between theelectrodes, the orientation of the molecules in the liquid crystal layerchanges, which alters the optical properties of the layer, in particularthe polarization of light traveling through the layer. Thus, the liquidcrystal layer in combination with one or more polarizing filters can beused to create an amplitude modulator (light valve). However, suchliquid crystal based devices have several disadvantages for SLMapplications. First, much of the light is absorbed in the polarizingfilters, reducing optical efficiency. In addition, the devices havelimited contrast ratio, (the ratio of the intensities of the pixel whenon and the pixel when off), and the response time of the most widelyused liquid crystals is very slow (several milliseconds). Liquidcrystals also have poor performance outside a fairly narrow temperaturerange. For these reasons and others, mechanical SLMs, which use movingstructures to deflect light, have been pursued.

[0007] An early mechanical SLM designed for use in a projection displaysystem is described by Nathanson, U.S. Pat. No. 3,746,911. Theindividual pixels of the SLM are addressed via a scanning electron beamas in a conventional direct-view cathode ray tube (CRT). Instead ofexciting a phosphor, the electron beam charges deflectable reflectiveelements arrayed on a quartz faceplate. Elements that are charged bendtowards the faceplate due to electrostatic forces. Bent and unbentelements reflect parallel incident light beams in different directions.Light reflected from unbent elements is blocked with a set of Schlierenstops, while light from bent elements is allowed to pass throughprojection optics and form an image on a screen.

[0008] Another electron-beam-addressed SLM is the Eidophor, described inE. Baumann, “The Fischer large-screen projection system (Eidophor)” 20J.SMPTE 351 (1953). In this system, the active optical element is an oilfilm, which is periodically dimpled by the electron beam so as todiffract incident light. A disadvantage of the Eidophor system is thatthe oil film is polymerized by constant electron bombardment and oilvapors result in a short cathode lifetime. A disadvantage of both ofthese systems is their use of bulky and expensive vacuum tubes.

[0009] A spatial light modulator in which movable elements are addressedvia electrical circuitry on a silicon substrate is described in K.Peterson, “Micromechanical Light Modulator Array Fabricated on Silicon”31 Appl. Phys. Let. 521 (1977). This SLM contains a 16 by 1 array ofcantilever mirrors above a silicon substrate. The mirrors are made ofsilicon dioxide and have a reflective metal coating. The space below themirrors is created by etching away silicon via a KOH etch. The mirrorsare deflected by electrostatic attraction: a voltage bias is appliedbetween the reflective elements and the substrate and generates anelectrostatic force. A similar spatial light modulator is thetwo-dimensional array described by Hartstein and Peterson, U.S. Pat. No.4,229,732. Although the switching voltage of this SLM is lowered byconnecting the deflectable mirror elements at only one corner, thedevice has low efficiency due to the small optically active area (as afraction of the entire device area). In addition, diffraction from theaddressing circuitry lowers the contrast ratio of the display.

[0010] A silicon-based micro-mechanical SLM in which a large fraction ofthe device is optically active is the Digital Mirror Device (DMD),developed by Texas Instruments and described by Hornbeck, U.S. Pat. No.5,216,537 and its references. The most recent implementations include afirst aluminum plate suspended via torsion hinges above addressingelectrodes. A second aluminum plate is built on top of the first andacts as a mirror. The double plate aluminum structure is required toprovide an approximately flat mirror surface that covers the underlyingcircuitry and hinge mechanism, which is essential in order to achieve anacceptable contrast ratio. The entire structure is made from aluminumalloys-the plates, torsion hinges and special “landing tips” each haveindependently optimized compositions. Aluminum can be deposited at lowtemperatures, avoiding damage to the underlying CMOS addressingcircuitry during manufacture. Aluminum has the disadvantage, however, ofbeing susceptible to fatigue and plastic deformation, which can lead tolong-term reliability problems and cell “memory”, where the restposition begins to tilt towards its most frequently occupied position.Additional disadvantages of the DMD include: 1) A large dimple (causedby the mirror support post) is present at the center of the mirror incurrent designs which causes scattering of the incident light andreduces optical efficiency. 2) The entire DMD structure is released viaplasma etching of a polymer sacrificial layer. This manufacturingprocess is problematic, in that it (a) requires large gaps betweenmirrors in order for the plasma etch release to be effective, and (b)pixel failures are created during the release process, which is notsufficiently gentle on the delicate micromirror structures. Due to thecomplex structure and process difficulties, commercialization of the DMDhas proceeded slowly.

[0011] Another SLM fabricated on a flat substrate is the Grating LightValve (GLV) described by Bloom, et. al., U.S. Pat. No. 5,311,360. Asdescribed in the '360 patent, the GLV's deflectable mechanical elementsare reflective flat beams or ribbons. Light reflects from both theribbons and the substrate. If the distance between the surface of thereflective ribbons and the reflective substrate is one-half of awavelength, light reflected from the two surfaces adds constructivelyand the device acts like a mirror. If this distance is one-quarter of awavelength, light directly reflected from the two surfaces willinterfere destructively and the device will act as a diffractiongrating, sending light into diffracted orders. A favored approach is tomake the device from ceramic films of high mechanical quality, such asLPCVD (low pressure chemical vapor deposition) silicon nitride.

[0012] Even though addressing circuitry cannot be placed below suchfilms, an inherent electromechanical bistability can be used toimplement a “passive” addressing scheme (Raj Apte, Grating Light Valvesfor High Resolution Displays, Stanford University Ph.D. thesis, June1994). The bistability exists because the mechanical force required fordeflection is roughly linear, whereas the electrostatic force obeys aninverse square law. As a voltage bias is applied, the ribbons deflect.When the ribbons are deflected past a certain point, the restoringmechanical force can no longer balance the electrostatic force and theribbons snap to the substrate. The voltage must be lowered substantiallybelow the snapping voltage in order for the ribbons to return to theirundeflected position. This latching action allows driver circuitry to beplaced off-chip or only at the periphery, and addressing circuitry doesnot need to occupy the optically active part of the array. In practice,this approach is difficult to implement: when the ribbon comes intocontact with the substrate, which is at a different potential, chargecan be injected into the insulating ceramic ribbon material, shiftingthe switching voltages and making passive addressing impossible. Filmnon-uniformity across the device can also shift the switching voltagessignificantly. Another problem with the GLV technology is sticking:since the underside of the deflected ribbons contacts the substrate witha large surface area, the ribbons tend to stick to the substrate. Filmscomprising the structure can be roughened, but this results inundesirable optical scattering, reducing the contrast ratio of thedevice.

[0013] Micro-mechanical mirror-based SLMs have an advantage overdiffraction-based SLMs because they reflect incident light at only oneangle, which can be quite large. This simplifies the design of theoptical system in which the modulated light may pass through the centerof the imaging lens, while maintaining high efficiency. This results inan image with fewer aberrations and lowers manufacturing cost.

[0014] The need therefore is for a spatial light modulator with a highcontrast ratio, high efficiency, high speed, which is easy to fabricate,and whose moving elements are made of reliable mechanical materials.

SUMMARY OF THE INVENTION

[0015] Briefly, in accordance with an embodiment of this invention, aspatial light modulator comprises an optically transmissive substrateand a circuit substrate. One or more reflective deflectable elements areattached to the lower surface of the optically transmissive substrate.This optically transmissive substrate is held above, and spaced apartfrom, a circuit substrate containing addressing circuitry capable ofselective activation of each reflective deflectable element.

[0016] In operation, individual reflective elements are selectivelydeflected and serve to spatially modulate light that is incident to, andthen reflected back through, the optically transmissive substrate.

[0017] In one embodiment of this invention, the spatial light modulatorcomprises an array of pixels. Each pixel comprises a single deflectablerigid mirror and a torsion hinge, which attaches the mirror to an upper,optically transmissive substrate. The optically transmissive substrateis held above a silicon substrate, on which is formed an array ofelectrodes. In one embodiment, an aperture layer is built into theoptically transmissive substrate to block light from reaching theelectrodes or the mirror support structure (hinges and attachments).Individual mirrors are selectively deflected electrostatically byapplying a voltage bias between individual mirrors and theircorresponding electrodes.

[0018] In accordance with an embodiment of this invention, a process forfabricating the spatial light modulator is provided. A sacrificial layeris deposited on a substrate. A hole is etched through the sacrificiallayer, the hole allowing for attachment of subsequent layers to theoptically transmissive substrate. A reflective layer is deposited on thesacrificial layer, and is patterned to define one or more reflectivedeflectable elements. The reflective layer is connected to thesacrificial layer through the hole. The sacrificial layer is removed sothat the reflective elements are free and may deflect. Addressingcircuitry and electrodes are formed on a circuit substrate. Thesubstrate and circuit substrate are aligned and joined such that thereflective elements may be selectively actuated by the addressingcircuitry and electrodes. The two substrates may be joined, for example,by epoxy around the periphery of the substrates.

[0019] In accordance with an embodiment of this invention, a processincludes asserting a bias voltage between the reflective deflectableelement and the addressing circuitry. The bias voltage may be changedduring device operation.

[0020] The electrical addressing circuitry on the silicon substrate maybe fabricated using standard CMOS technology, and resembles alow-density memory array.

[0021] Since the two substrates are joined together only after they areindividually fabricated, the fabrication processes for each substrateare decoupled. As there is no concern for CMOS compatibility during themanufacturing of the top substrate, an advantage of the spatial lightmodulator of this invention is that the mechanically deflectablereflective elements can be made from materials chosen only for theirexcellent mechanical properties, such as LPCVD-deposited siliconnitride, silicon oxide, amorphous silicon and poly-silicon. Since thesefilms are deposited at high temperatures, they are not normallycompatible with CMOS processes, because the latter use aluminuminterconnects which would melt at these higher temperatures.

[0022] A further advantage of this spatial light modulator is that afterthe two substrates are bonded together, the moving parts may be fullyencapsulated. This provides an excellent method of packaging and leadsto high device robustness.

[0023] The spatial light modulator of this invention has the furtheradvantage that it is inexpensive and straightforward to construct. It iscomposed of two substrates: one which may be made using standard CMOStechniques, and a second optically transmissive substrate containing thedeflectable reflective elements, which is very simple to fabricate.

[0024] Yet another advantage of this spatial light modulator is that alight blocking aperture layer, as well as other planar optics (e.g.color filters, reflectivity enhancement coatings, micro-lenses) can beincorporated into the optically transmissive substrate. This can improvethe contrast ratio and increase the effective light deflection angle,and reduce the cost of free-space optics at the systems level.

[0025] Yet another advantage of this spatial light modulator is that themotion limiting structures can also be made of high-temperaturematerials which are hard and have long lifetimes. Because of theirhardness and geometry, the motion limiting structures have a smallcontact area during operation, which greatly reduces sticking forcesbetween the structures and the substrate. Also, the motion limitingstructures are at the same electrical potential as the substrate withwhich they come into contact which prevents sticking via welding andcharge injection. These were problems encountered with early versions ofthe DMD and the GLV.

[0026] Yet another advantage of this spatial light modulator is that thehigh-temperature processing of the optically transmissive substrateallows for the deposition of dielectric films with alternating high-lowindices of refraction onto the deflectable reflective elements, whichenhance their reflectivity.

[0027] These and other advantages will become apparent to those skilledin the art after consideration of the ensuing drawings and detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 shows a top perspective view of a corner of an embodimentof a spatial light modulator of the present invention.

[0029] FIGS. 2A-2F show a bottom perspective view of a pixel cell ofFIG. 1 during several stages of fabrication.

[0030]FIGS. 3A and 3B show a cross-section of a pixel cell of FIG. 1modulating a light beam.

[0031]FIG. 4 shows a graph of hysteresis in the deflection angle of themirror of FIG. 1 versus applied voltage bias.

[0032]FIG. 5 shows a graph of the electrical and mechanical torquesacting on a deflectable mirror for several different bias voltages.

[0033]FIG. 6A shows a DRAM structure for individually addressing the SLMpixel cells of FIG. 1.

[0034]FIG. 6B shows an SRAM structure for individual addressing the SLMpixel cells of FIG. 1.

[0035]FIG. 7 shows a top view of a spacer placement in a dense pixelarray.

[0036] FIGS. 8A-8H show bottom views of mirror arrays with differenthinge designs.

[0037] FIGS. 9A-9D show the fabrication process of a pixel cell havingthe hinge between the mirror and optically transmissive substrate(sub-hinge design).

[0038] FIGS. 10A-10D show embodiments of the sub-hinge design.

[0039] FIGS. 11A-11C show the fabrication process of a pixel cell havingthe mirror between the hinge and optically transmissive substrate(super-hinge design).

[0040]FIG. 12 shows an embodiment of the super-hinge design.

[0041]FIG. 13 shows an exploded top perspective view of a corner of anembodiment of a spatial light modulator of the present invention.

[0042]FIG. 14 shows a cell having the sub-hinge design of FIG. 10Aconfigured in an array of similarly structured cells. REFERENCE NUMERALSIN THE DRAWINGS 10 Micro-mechanical spatial light modulator (SLM) 12Pixel cells 14 Lower surface 16 Upper surface 20 Optically transmissivesubstrate 22 Aperture layer 24 Protective layer 25 Hole 26 Sacrificiallayer 28 Mirror structural support layer 30 Hinge layer 32 Reflectivelayer 34 Circuit substrate 36 Addressing circuitry 38 Passivation layer42 Bottom electrode 43 Contact 44 Spacer 46 Passivation layer 48 Mirror49 Motion stop 50 Hinge 51 Hinge support 54 Attachment region 56Incoming light beam 58 Outgoing light beam 60 Word line 62 Bit line 64Light source 66 Imaging optics 68 Transistor 70 First dielectric layer72 Second dielectric layer 74 Voltage source 78 Optical dump 111 Bumps

DETAILED DESCRIPTION OF THE INVENTION

[0043] This description refers to several figures which containreference numerals. The same reference numerals in different figuresindicate similar or identical items.

[0044] Throughout this description, the words “optical” and “light” areused. In the description and claims, “optical” means related to anyelectromagnetic frequencies, not just frequencies in the visible range.For example, an “optically transmissive substrate” is a substrate whichis transmissive to electromagnetic propagation of a working frequency,whether in the visible range or not.

[0045] A top perspective view of a corner of an embodiment of amicro-mechanical spatial light modulator 10 (hereinafter, “SLM 10”) ofthis invention is shown in FIG. 1. An exploded view of the SLM 10 ofFIG. 1 is shown in FIG. 13. SLM 10 may include pixel cells of anyconfiguration or array size. However, for clarity, only four pixel cells12, 12 a, 12 b and 12 c in a two by two grid configuration are shown inFIG. 1. The pixel cells 12, 12 a, 12 b and 12 c have a pixel pitch of,for example, 12 microns. “Pixel pitch” is defined as the distancebetween like portions of neighboring pixel cells.

[0046] Reflective deflectable elements (e.g., mirrors 48, 48 a, 48 b and48 c), each corresponding to a respective pixel cell 12, 12 a, 12 b and12 c, are attached to the lower surface 14 of an optically transmissivesubstrate 20 in an undeflected position. Thus, mirrors 48, 48 a, 48 band 48 c are visible through optically transmissive substrate 20 inFIG. 1. For clarity, light blocking aperture layers 22, between themirrors 48, 48 a, 48 b or 48 c and the optically transmissive substrate20, are represented only by dashed lines so as to show underlying hinges50, 50 a, 50 b and 50 c. The distance separating neighboring mirrors maybe, for example, 0.5 microns or less.

[0047] One process for fabricating SLM 10 is illustrated in bottomperspective view in FIGS. 2A-2F. For clarity, only the fabrication ofpixel cell 12 is described. However, from this description, it will beapparent that pixel cells 12 a, 12 b, 12 c and the other pixel cells inSLM 10 may be fabricated at the same time and in the same manner aspixel cell 12 is fabricated.

[0048] The optically transmissive substrate 20 is made of materialswhich can withstand subsequent processing temperatures. The opticallytransmissive substrate 20 may be, for example, a 4 inch quartz wafer 500microns thick. Such quartz wafers are widely available from, forexample, Hoya Corporation U.S.A. at 960 Rincon Circle, San Jose, Calif.95131.

[0049] As seen in FIG. 2A, a light blocking layer (e.g., a 50 nm thicktungsten layer) is deposited and patterned to form the light-blockingaperture layer 22. The aperture layer 22 is made out of an opaquematerial (e.g., tungsten) which remains stable during subsequentfabrication steps. The tungsten may be deposited using, for example,well-known sputtering techniques. A pattern of photoresist is formedover the aperture layer 22 using well-known photolithographic processes.Aperture layer 22 is then etched using a Drytek 100 plasma etcher. Amixture of 50% by volume SF₆ and 50% by volume C₂ClF₅ is introduced intothe reaction chamber of the etcher at a rate of 300 sccm (150 sccm forHF₆ and 150 sccm for C₂ClF₅). Etching occurs at a pressure ofapproximately 100 mTorr with the power setting on the etcher at 500watts until the optically transmissive substrate 20 is exposed(approximately one minute). After etching, the remaining photoresist isremoved using a conventional oxygen plasma strip. Patterning describedhereinafter may be similarly performed.

[0050] As seen in FIG. 2B, an optically transmissive protective layer 24(e.g., an approximately 94nm thick 7%-by-weight phosphorus-doped silicondioxide) is next deposited as a passivation layer. The reflectivedeflectable element (mirror 48) is to be connected to opticallytransmissive substrate 20 through protective layer 24. The silicondioxide protective layer 24 may be deposited, for example, by LPCVDprocesses in the quartz tube of a Tylan furnace at approximately 400° C.and 250 mTorr for approximately 5 minutes. SiH₄, O₂, and PH₃ areintroduced into the chamber at rates of 28, 115, and 7 sccm,respectively. The phosphorous-doped silicon dioxide is then reflowed at1100° C. for 20 minutes in a steam environment.

[0051] A sacrificial layer 26 (e.g., an approximately 0.6 μm thickamorphous silicon layer), which will eventually be removed as describedhereinafter, is deposited on the protective layer 24. The amorphoussilicon layer may be deposited using LPCVD processes in, for example,the quartz tube of a Tylan furnace. The SLM 10 is exposed in the quartztube at approximately 670° C. and 200 mTorr for 135 minutes. Acomposition of SiH₄ and H₂ is introduced into the quartz tube at a flowrate of 246 sccm (146 sccm for SiH₄ and 100 sccm for H₂).

[0052] Holes 25 are patterned through sacrificial amorphous siliconlayer 26 by selective anisotropic etching by using, for example,patterned plasma etching in a 50% SF₆ and 50% C₂ClF₅ (by volume)environment until a portion of protective layer 24 is exposed throughsacrificial layer 26. Such etching may occur in the reaction chamber ofa Drytek 100 plasma etcher. The gas composition is introduced at a rateof 100 sccm (50 sccm for SF₆ and 50 sccm for C₂ClF₂), and a pressure of150 mTorr. Typically, it takes approximately 4.5 minutes to expose theportion of protective layer 24 through sacrificial layer 26 under theseconditions.

[0053] A mirror structural support layer 28, for example anapproximately 138 nm thick low-stress silicon nitride layer, isdeposited and patterned to form mirror 48 and motion stop 49. Mirror 48is a substantially rigid plate. The low stress silicon nitride layer maybe deposited, for example, in a quartz tube of a Tylan furnace by usingLPCVD processes at approximately 785° C. and 250 mTorr for approximately36 minutes. Deposition occurs, for example, by introducing Sicl₂H₂ andNH₃ into the quartz tube at 165 sccm and 32 sccm, respectively. Afterdeposition and patterned light exposure of photoresist, the siliconnitride may be etched using an AMT 8100 hexagonal-electrode plasmaetcher powered at 1200 watts. The etch gases, for example, O₂ and CHF₃,are introduced into the reaction chamber at respective flow rates of 6sccm and 85 sccm, respectively, with an etch period of 17 minutes. Underthese conditions, the polysilicon to silicon nitride selectivity ratiois approximately 1:6.

[0054] As seen in FIG. 2C, a hinge layer 30 (e.g., an approximately 40nm thick layer of low-stress silicon nitride) is then grown andpatterned to additionally define the torsion hinge 50 (a top view ofthis pattern can be seen in FIG. 8A). At least a portion of hinge 50contacts protective layer 24 through holes 25 to define supports 51(FIGS. 2D-2F). The hinge 50 operates by “torsion” which means that thehinge 50 is twisted by applying torque about the longitudinal directionof the hinge 50. Thus, the end of hinge 50 attached to the mirror 48 isangularly deflected with respect to the ends supported by supports 51and 51. Hinge 50 may be, for example, approximately 0.5 microns wide.

[0055] The thin layer of low stress silicon nitride for the hinge layer30 is deposited in a quartz tube of a Tylan furnace using an LPCVDprocess. Sicl₂H₂ and NH₃ are introduced into the quartz tube at a flowrate of, for example, 165 sccm and 32 sccm, respectively. The depositionoccurs, for example, at a temperature of 785° C. and at a pressure of250 mTorr for 11 minutes.

[0056] As shown in FIG. 2D, the sacrificial layer 26 is then partiallyremoved using an isotropic etch process. The etch process is isotropicso that portions of the sacrificial layer 26 are removed from underneaththe mirror 48 and hinge 50. After the partial etch of sacrificial layer26, the sacrificial layer 26 that is not underneath mirror 48 and hinge50 is removed. On the other hand, significant portions of thesacrificial layer 26 underneath mirror 48 and hinge 50 remain due to theprotection of mirror 48 and hinge 50. Therefore, after the partial etch,sacrificial layer 26 continues to support mirror 48 and hinge 50 andprevents airborne particulates from lodging underneath mirror 48 andhinge 50 during further fabrication steps described hereinafter. Onesuitable isotropic etch process is by exposure to a plasma etchingprocess in the reaction chamber of a Drytek 100 plasma etcher.Approximately 100% SF₆ is introduced into the reaction chamber at a flowrate of approximately 50 sccm with the power setting on the etcher setat 375 watts. Etching occurs for approximately 100 seconds at roomtemperature (however, the plasma generates heat), and a pressure ofapproximately 150 mTorr. In this process, the selectivity ratio ofsilicon to silicon nitride is approximately 6:1.

[0057] Referring to FIG. 2E, horizontal surfaces (e.g., mirrorstructural support layer 28, hinge layer 30, and portions of protectivelayer 24) of the SLM 10 are then coated with a conductive and reflectivelayer 32 (e.g., approximately 30 nm thick layer of aluminum) which isoptically reflective. Some vertical surfaces (e.g., the vertical surfaceof hinge 50 proximate the mirror 48) are also coated to electricallyconnect the reflective layer 32 on the mirror structural support layer28 with the reflective layer 32 on the protective layer 24. For clarity,the portions of reflective layer 32 on hinge layer 30 and the verticalsurfaces are not shown in FIG. 2E. Such a reflective layer 32 may bedeposited by, for example, evaporating aluminum downwardly at an anglesuch that the horizontal vector of the angle is from mirror 48 to motionstop 49. With this angle, no metal (aluminum) exists on protective layer24 at the point where motion stop 49 contacts protective layer 24because motion stop 49 shields this surface from metal deposition. Notethat the protective layer 24 is exposed due to the partial etching ofsacrificial layer 26 described above. The evaporation may occur, forexample, in the reaction chamber of an e-gun thermal evaporator at adeposition rate of one nanometer per second.

[0058] Spacers 44 (FIGS. 1 and 13) are provided on the opticallytransmissive substrate. Spacers 44 are, for example, composed ofHoechst-Delanese AZ4330-RS photoresist, spun on at 5000 rpm for 30seconds, exposed and patterned to form spacers 44 using conventionalphotolithographic techniques, then hard baked at 233° C. for 1 hour togive increased structural rigidity.

[0059] The mirrors 48 a, 48 b and 48 c are fully released from opticallytransmissive substrate 20, except at hinge supports 51 and 51, with asecond isotropic etch, for example, a xenon diflouride etch process,which completely removes the sacrificial layer 26. This etching isperformed at approximately 4 Torr in an approximately 100% xenondiflouride environment for approximately 20minutes at room temperature.Under these conditions the selectivity of this etching process is over ahundred to one.

[0060] The optically transmissive substrate 20 with the mirror arrayattached thereto is now ready to bond to a circuit substrate 34 (e.g., asemiconductor substrate) containing addressing circuitry 36, as shown incross section in FIG. 3A. Spacers 44 (FIGS. 1 and 13) are bonded to thecircuit substrate 34 to hold optically transmissive substrate 20 apartfrom, but in close proximity to, circuit substrate 34.

[0061] In one embodiment, planar optical elements such as two dielectriclayers 70 and 72 (FIG. 2F) having a different index of refraction aredeposited as mirror structural support layer 28. This stack ofdielectric layers may reflect light or filter out specific frequencyranges. For example, a layer of silicon dioxide (optical index of 1.46)deposited on top of a layer of silicon nitride (with an optical index of2.0) will enhance the reflectivity of, for example, aluminum reflectivelayer 32 with a reflectivity of 92% to 95% over much of the opticalspectrum if the silicon nitride layer is 68nm thick and the silicondioxide layer is 96nm thick.

[0062] After sacrificial layer 26 is fully etched away, opticallytransmissive substrate 20 is bonded to the circuit substrate 34. First,the substrates 20 and 34 are optically aligned and held together, andcan be glued together with epoxy dispensed around the edge of circuitsubstrate 34. Since the top substrate 20 is optically transmissive,alignment can be accomplished easily by aligning a pattern on theoptically transmissive substrate 20 to a pattern on the circuitrysubstrate 34. By dispensing epoxy around the edges of opticallytransmissive substrate 20 and circuit substrate 34 in a cleanenvironment, the mirror 48 may be isolated from airborne particulates.

[0063] In FIG. 3A, a bottom electrode 42 (e.g., a 500 nm thick aluminumbottom electrode) of cell 12 is shown connecting to addressing circuitry36 through contact 43. Many configurations are possible. In oneembodiment, the active bottom electrode 42 should be physically locatedhigher than the rest of the circuit components 36 and interconnects. Inthis embodiment, the bottom electrode 42 interacts with the overhangingmirror 48 through electrostatic forces.

[0064] The operation of the above-described embodiment is shown in FIG.3A and FIG. 3B. In FIG. 3A, the mirror 48 is undeflected. In thisunbiased state, an incoming light beam, from a light source 64,obliquely incident to SLM 10 passes through the optically transmissivesubstrate 20 and is reflected by the flat mirrors 48 and partiallyreflected by aperture layer 22. The angle of the outgoing light beam 58is thus also oblique to the optically transmissive substrate 20. Theoutgoing light beam may be received by, for example, an optical dump 78.The incorporation of the aperture layer 22 into the opticallytransmissive substrate 20 is a technique to eliminate unwanted lightscattering from the underlying hinge 50.

[0065] Cell 12 with a voltage bias applied between the mirror 48 and thebottom electrode 42 applied is shown in FIG. 3B. The mirror 48 isdeflected due to electrostatic attraction. Because of the design of thehinge 50, the free end of the mirror 48 is deflected towards the circuitsubstrate 34. Note that hinge 50 may be more flexible than mirror 48such that the application of force causes substantially all of thebending to be in hinge 50. This may be accomplished by making hingelayer 30 much thinner than mirror structural support layer 28 asdescribed above. The deflection of the mirror 48 deflects the outgoinglight beam 58, by a significant angle, into the imaging optics 66.

[0066] The motion of mirror 48 is limited by motion stop 49 contactingthe protective layer 24 deposited on optically transmissive substrate 20(see FIG. 3B) so that mirror 48 does not contact the circuit substrate34. Since contact does not occur, the electrically connected mirrors 48,48 a, 48 b and 48 c remain at the same potential. Also, there is nocharge injection and welding between the mirror 48 and the electrode 42which can result in sticking. When mirror 48, in the undeflectedposition, is separated from optically transmissive substrate 20 by, forexample, 2.8 microns, the motion stop 49 may extend (for example,approximately 3.3 microns) from the pivot axis of hinge 50.

[0067] The full electromechanical characteristics of the modulator arefurther elucidated in FIG. 4 and FIG. 5. In FIG. 4, deflection angle αof the mirror 48 is plotted against the voltage bias and hysteresis isobserved. As a voltage bias is applied between mirror 48 and electrode42 (FIGS. 3A and 3B), the mirror 48 deflects (see line 401 of FIG. 4).When the mirror 48 deflects past the snapping voltage V_(snap) (e.g.,approximately 6.8 volts), the restoring mechanical force of the hinges50 can no longer balance the electrostatic force and the mirror 48 snapstoward the electrode 42 of the circuit substrate 34 (see line 402 ofFIG. 4) until motion stop 49 contacts optically transmissive substrate20. The voltage must be lowered substantially below the snapping voltage(see line 403 of FIG. 4) to V_(release) (e.g., approximately 5.6 volts)in order for the mirror 48 to return towards its undeflected position(see line 404 of FIG. 4). Thus, the mirror 48 would be anelectromechanically bistable device between voltages V_(release) andV_(snap). In other words, given a specific voltage between V_(release)and V_(snap) there are two possible deflection angles α of mirror 48depending on the history of mirror 48 deflection. Therefore, mirror 48deflection acts as a latch. These bistability and latching propertiesexist since the mechanical force required for deflection is roughlylinear with respect to deflection angle α, whereas the opposingelectrostatic force is inversely proportional to the distance betweenmirror 48 and electrode 42.

[0068] This latching action allows driver circuitry to be placedoff-chip or only at the periphery using passive addressing instead ofhaving a memory cell for driving each electrode. For example, eachelectrode 42 in each given row may be electrically connected while eachmirror 48 in each given column is electrically connected. Duringaddressing, for each pixel cell not in the same row or column as theaddressed pixel cell, the applied voltage bias is at an intermediatevoltage (e.g., 6.2 volts) between V_(release) and V_(snap). Thus, forthese pixel cells, the deflection of mirror 48 represents a one binarystate (e.g., a binary one) if the mirror 48 is deflected at line 403 andthe other binary state (e.g., a binary zero) if the mirror is deflectedat line 401. In other words, this intermediate voltage does not uniquelydetermine the state of mirror 48 deflection.

[0069] If an on state (or an off state) is to be programmed at theaddressed pixel cell, the electrode 42 voltage of the addressed pixelcell row is altered to increase (or decrease to turn off) the appliedbias voltage. The mirror 48 voltage of the addressed pixel cell columnis also altered to increase (or decrease to turn off) the applied biasvoltage. For unaddressed pixel cells that happen to be in the same rowor column as the addressed pixel cell, the applied bias voltageincreases (or decreases to turn off), but is still between V_(release)and V_(snap). Therefore, the binary states do not change for theunaddressed pixel cells that are in the same row and column as theaddressed pixel cell. However, for the addressed pixel cell, both theelectrode 42 and mirror 48 voltages have been altered to increase (ordecrease to turn off) the applied bias voltage. This increase is greaterthan V_(snap) (or the decrease is less than V_(release) to turn off theaddressed pixel) and thus the addressed pixel cell is on (or off). Inorder to address and program, only one driver circuit for each row andcolumn is needed. Therefore, the driver circuits may be placed along theperiphery of the device or off chip.

[0070] Even for fully active addressing in which each electrode 42 has adriving circuit (such as a transistor in a DRAM configuration),connecting mirrors in groups could increase addressing efficiency. Thismay be accomplished either with connections at the periphery of themirror array, or by depositing pillars connecting the mirrors to thecircuit substrate at pixel locations. Since the electrostatic forcedepends only on the total voltage between conductive and reflectivelayer 32 and bottom electrode 42, a negative voltage applied to a mirrorgroup (via reflective layer 32) reduces the operating voltage of thecorresponding electrodes thus reducing the voltage requirement of SLM10. It is desirable, for example, to keep the operating voltage below 5Vbecause 5V switching capability is standard to the semiconductorindustry. In addition, the amount of charge needed to bias eachelectrode of the addressed pixel is smaller than an embodiment in whichall mirrors are held at ground. Thus the time required to program theaddressed pixel cell is relatively fast.

[0071] In FIG. 5, we plot mechanical and electrical torques vs.deflection angle α as the applied voltage bias is increased and themirror 48 tilts. As shown in FIG. 5, the mechanical torqueτ_(mechanical) caused by the mechanical restoring force of the hinge 50is roughly linear relative to the deflection angle α. On the other hand,each electrical torque (τ_(electrical)) curve caused by theelectrostatic force between the mirror 48 and electrode 42 obeys aninverse square law and rises sharply with increasing deflection angle α(as the capacitance of the mirror 48—electrode 42 structure isincreased). At low voltage biases, as exemplified by bottom curve(V=V_(a)), there is an equilibrium point α_(E). If the mirror 48 isslightly more (or less) tilted than the equilibrium point α_(E), theupward directed mechanical force (or the downward directed electrostaticforce) dominates and the mirror 48 deflects back up (or down) to theequilibrium point α_(E). By changing the on-state voltage bias betweenthe mirror 48 and electrode 42, the tilt of the mirror 48 is controlled.

[0072] If the voltage bias between mirror 48 and electrode 42 exceeds acritical value (here V=V_(b) as seen in the middle curve), theequilibrium point α_(E) no longer exists and the mirror 48 snaps towardthe circuit substrate 34 (see line 402 of FIG. 4). Snapping occurs whenthe mirror 48 is approximately half-way deflected towards the circuitsubstrate 34 if the mechanical torque is linear in angle. If noalternate stopping mechanisms were in place, the snapping action wouldcontinue until mirror 48 makes contact with electrode 42. It may bedesirable to avoid this mode of operation because sticking might occurdue to welding. Welding is particularly likely when the surfaces makingcontact are originally at different electrical potentials, or when largecontact surface areas are in play as occur with malleable materials suchas metal.

[0073] The motion stops 49 described above are made of hard materialssuch as silicon nitride. These hard materials have potentially longerlifetimes than metal structures. Motion stops 49 also have a limitedcontact area with the optically transmissive substrate 20 and thereforereduce sticking forces. By keeping the motion stops 49 at the samepotential as the reflective layer 32 with which they come into contact,electrical potential differences that lead to welding can also beavoided. Snapping and thus physical contact between motion stops 49 andoptically transmissive substrate 20 can be avoided entirely by keepingV<V_(b).

[0074] If the SLM 10 is operated at voltages past the snapping point, itcan be operated in a digital manner using either active addressing(i.e., a separate transistor drives electrode 42 at each pixellocation), or using passive addressing (i.e., only one driver circuitfor each row or column), by exploiting the electromechanical bistabilitymentioned earlier. If SLM 10 operates at voltages greater than V_(snap),deflection along line 403 may represent one binary state while all otherdeflections represent the other binary state.

[0075] If the SLM 10 is operated at voltages below the snapping point,it can be operated in an analog fashion using active addressing. Forexample, for different deflection angles α, a different intensity oflight may be reflected to imaging optics 66 if light source 64 emitsrays from a wide range of locations. The use of high quality mechanicalmaterials described above results in good uniformity over the pixelarray, and makes analog operation practical. The mirror 48 deflectionwill then be proportional to the charge stored at each correspondingelectrode. Operation below the snapping point also has the advantage ofpreventing mechanical contact during operation, eliminating possiblesticking problems.

[0076] For mirror operation past the snapping voltage, it is furtherpossible to vary the addressing voltage as a function of time asfollows. During the active addressing stage, the addressing is set tothe level required for electrostatic-force-based mirror deflection forthose electrodes where mirror deflection is required. After the mirrorsin question have deflected, the voltage required to hold in thedeflected position is less than that required for the actual deflection.This is because the gap between the deflected mirror and the addressingelectrode is already smaller than when the mirror is in the process ofbeing deflected. Therefore, in the stage after the active addressingstage, (called the “hold stage”, for example), the addressing voltagelevel could be reduced from its original level without substantiallyaffecting the state of the mirrors. One advantage of having a hold stagevoltage is that the undeflected mirrors are now also subject to asmaller electrostatic attractive force than before, and they thereforeattain a position closer to the zero-deflected position. This improvesthe optical contrast ratio between the deflected mirrors and theundeflected mirrors.

[0077] An electrical schematic of a memory array portion of addressingcircuitry 36 is shown in FIG. 6A and FIG. 6B. If active addressing isemployed, an addressing scheme embodied in the circuitry of FIG. 6A canbe used to address each pixel cell of the SLM 10 individually.Substrates 20 and 34 are not shown in FIG. 6A, and the mirror 48 andbottom electrode 42 are drawn symbolically. The scheme is identical tothat used for a DRAM (dynamic random access memory). Each pixel cell 12,12 a, 12 b and 12 c is driven by a respective NMOS transistor 68, 68 a,68 b and 68 c. For example, if pixel cell 12 is to be addressed,electrode 42 is charged as follows. The state of the correspondingcolumn of pixels (containing pixel cells 12 and 12 c) is set by holdingthe corresponding bit line 62 at the appropriate bias voltage for thedesired mirror deflection. The bias is relative to the mirrors 48, whichare connected to a common voltage such as ground. The corresponding wordline 60 is then pulsed low-high-low (i.e., NMOS transistor 68 istemporarily opened) and the voltage value is stored as charge betweenthe bottom electrode 42 and mirror 48. An additional capacitor may beplaced electrically in parallel to the mirror-electrode combination toinsure that enough charge is stored to overcome leakage.

[0078] Another embodiment uses an SRAM (static random access memory)type cell to drive the actuating electrodes (FIG. 6B). For example,pixel cell 12 is addressed by applying a voltage representing a binaryone on the corresponding bit line 62. The voltage is sufficient tocharge electrode 42 and deflect mirror 48. A voltage representing abinary zero is present on the other corresponding bit line 62 (bar). Thecorresponding word line 60 is selected by asserting a voltage sufficientto open transistors 69 a and 69 b. The input to inverter 69 c and theoutput from inverter 69 d represent a binary zero. The output frominverter 69 c and the input to inverter 69 d represents a binary one.With transistor 69 a open, electrode 42 is charged through bit line 62.

[0079] Since the mirror 48 area may be relatively large on semiconductorscales (12×12 microns=144 square microns), more complex circuitry can bemanufactured beneath each actuating electrode. Possible circuitryincludes, but is not limited to, storage buffers to store timesequential pixel information at each pixel; and electronic circuitry tocompensate for possible non-uniformity of mirror/electrode separation bydriving the electrodes at varying voltage levels.

[0080] With the appropriate choice of dimensions (substrate 20 and 34separation of 1 to 5 μm and hinge thickness of 0.03 to 0.3 μm) andmaterials (silicon nitride), an SLM 10 can be made to have an operatingvoltage of only a few volts. The angular torsion modulus of hinge 50 maybe, for example, approximately 3.3×10⁻¹⁴ Newton meters per degree ofrotation. As discussed above, the voltage at which the addressingcircuitry must operate can be made even lower by maintaining the mirror48 potential negative (or positive), as referenced to the circuit ground(the bias voltage). For example, in the negative bias case, this has theeffect of shifting the hysteresis curve of FIG. 4 to the left, so thatthe actuating electrode array can operate in a low voltage range such as0-5V and cause mirror deflection. This results in a larger difference indeflection angle for a given voltage. The maximum negative bias voltageis −V_(release). The negative voltage may be asserted to mirror 48 by,for example, closing switch 76 coupling the mirror 48 to a voltagesource 74 configured to assert a negative voltage (see pixel cell 12 ofFIG. 6A).

[0081] Depending on the planarity and resistance to bending of the twosubstrates 20 and 34, spacers 44 may need to be embedded in the mirrorarray itself. FIG. 7 shows a top view of a reasonably contiguous mirrorarray having a spacer 44 in the middle. The mirror array includes 56mirrors 48, 48 a to 48 z, 48 aa to 48 az, 48 ba, 48 bb and 48 bc. Forclarity, optically transmissive substrate 20 and circuit substrate 34are not shown and each mirror 48 is represented as a square. Spacer 44is centered among mirrors 48 aa, 48 ab, 48 ai and 48 aj, each mirrorhaving an edge coplanar with a corresponding edge of spacer 44 as shownin FIG. 7.

[0082]FIG. 8A shows a top plan view of pixel cells 12 and 12 a of theSLM 10 created by the process described with reference to FIGS. 2A-2D.The mirrors 48 and 48 a rotate around the axis defined by the thinhinges 50 and 50 a. Mirror 48 and 48 a motion is limited by the motionstops 49 and 49 a, which move towards and eventually hit the opticallytransmissive substrate 20 to which the mirrors 48 and 48 a are attached(see FIG. 3B). In one embodiment, the diagonal lines represent the areawhich includes a relatively thick silicon nitride layer as compared tothe thinner hinges. This reinforcement mechanically stiffens mirrors 48and 48 a while retaining flexibility in hinges 50 and 50 a. Similarreinforcement is seen in FIGS. 8B-8E.

[0083] There exist many possible variations in the design of the mirror48 that constitute the optically active component of the SLM 10. FIGS.8A-8D show variations in which motion stop 49 and mirror 48 aresubstantially coplanar. One embodiment has two motion stops 49 _(8B) isshown in FIG. 8B. In FIG. 8C, hinges 50 _(8C) are connected directly tomotion stops 49 _(8C). The embodiments of FIG. 8C and 8D are similarexcept that FIG. 8D shows only one motion stop 49 _(8D). FIG. 8E showssupports 51 _(8E) that are adjacent. Pixel cell 12 _(8E) of FIG. 8E hasno motion stops at all and is most useful if SLM 10 operates only atbelow V_(snap).

[0084] In the embodiments shown in FIGS. 8F and 8G, the hinges 50 _(8F)and 50 _(8G) operate by flexure and not by torsion. “Flexure” means thatthe ends of hinges 50 _(8F) and 50 _(8G) are fixed and that angulardeflection of mirrors 48 _(8F) and 48 _(8G) causes hinge 50 _(8F) and 50_(8G) to deflect angularly at a middle portion of hinges 50 _(8F) and 50_(8G), thereby causing hinges 50 _(8F) and 50 _(8G) to stretch along thelongitudinal direction of hinges 50 _(8F) and 50 _(8G). The hinges 50_(8F) and 50 _(8G) of FIG. 8F and FIG. 8G have hinge supports 51 _(8F)and 51 _(8G) which tie hinges 50 _(8F) and 50 _(8G) down to opticallytransmissive substrate 20 (FIGS. 1, 2A, 2B, 2C, 2D, 2E, 2F, 3A, 3B, 9A,9B, 9C, 9D, 10A, 10B, 10C, 11A, 11B, 11C, 12 and 13). Thus hinges 50_(8F) and 50 _(8G) bend longitudinally and not torsionally. In thisembodiment of hinges 50 _(8F) and 50 _(8G), the mechanical restoringforce will increase with faster-than-linear dependence on deflection, asthe strain in primarily tensile. A hinge 50 _(8F) or 50 _(8G) with thischaracteristic might be useful when the mirror 48 is operated in ananalog manner, since the snapping angle (and thus V_(snap)) will beincreased. In FIG. 8H, the hinge 50 _(8H) is a cantilever design, alsooperating by flexion and not by torsion.

[0085] A second fabrication process to produce the micro-mechanical SLM10 of this invention is illustrated in cross-section in FIGS. 9A-9D andFIG. 10A. This process uses multiple silicon nitride layers to achieve amirror-type structure with a higher aperture ratio (fraction ofoptically active area) than is possible with the process outlined inFIGS. 2A-2F. This is partly because the mirror stop 49 _(10A) (FIG. 10A)and mirror 48 _(10A) (FIG. 10A) lie in different planes. Opticallytransmissive substrate 20 is made of materials such as quartz which canwithstand subsequent processing temperatures. In this process, thedeposition of the light-blocking aperture layer 22 and protective layer24, shown in FIGS. 1, 2A, 2B and 2C, has been skipped but could be addedas the first step of the process.

[0086] Sacrificial layer 26 _(10A) (e.g., an approximately 0.5 micronthick, LPCVD-grown amorphous silicon layer) is deposited. After holes 25_(10A) are patterned through to optically transmissive substrate 20 asseen in FIG. 9A, a motion stop layer (e.g., a 150 nm thick LPCVD-grownlow-stress silicon nitride layer) is deposited and patterned to form themotion stop 49 _(10A) having a sharp contact tip 90.

[0087] Next, a hinge layer (e.g., a 40 nm thick layer of low-stresssilicon nitride) is grown and then patterned to define the torsionhinges 50 _(10A) as seen in FIG. 9B. A second sacrificial layer 27 isdeposited (e.g., an approximately 0.5 micron thick, LPCVD-grownamorphous silicon layer), and patterned so that a hole 25 c reaches downto the hinge 50 _(10A) (FIG. 9C). This second sacrificial layer 27_(10A) could be polished with commonly known chemical mechanicalpolishing (CMP) techniques to achieve a flat surface for subsequent filmdepositions. Since subsequently deposited layers include the mirrorstructural support layer 28 _(10A), the mirror structural support layer28 _(10A) will feature enhanced flatness thus improved reflectiveuniformity and improved system contrast and brightness. Finally, anapproximately 138 nm thick silicon nitride mirror structural supportlayer 28 _(10A) is deposited and patterned to form the substantiallyrigid mirror plate (FIG. 9D).

[0088] Next, sacrificial layers 26 _(10A) and 27 are partially removedusing an isotropic (e.g., a xenon diflouride gas etch; the 100% SF₆plasma process refered to earlier may also be used) etch process, andthe entire structure is coated with, for example, a very thin layer (30nm) of aluminum (reflective layer 32 _(10A) of FIG. 10A) which is bothhighly reflective and serves to electrically connect the mirrorstogether as described above.

[0089] Finally the mirrors are fully released with a second isotropicetch process (for example, a xenon diflouride gas etch), completelyremoving the sacrificial layer 26 _(10A). The mirrors are now ready tobe joined with the circuit substrate 34 containing addressing circuitry,using, for example, the same techniques described earlier in referenceto FIG. 2 and FIG. 3. Thus a sub-hinge structure is fabricated in whicha hinge 50 _(10A), which may be transparent, is disposed between theoptically transmissive substrate 20 and the mirror 48.

[0090] FIGS. 10A-10C show embodiments of the sub-hinge structurefabricated using the process above. For clarity, SLM's 10 _(10A-10D) arerotated 90 degrees so that hinges 50 _(10A-10D) may be seen. FIG. 10Ashows a cell 12 _(10A) with a torsion hinge 50 _(10A) and one motionstop 49 _(10A) centrally located. This device is shown to scale in anarray of similarly structure cells in FIG. 14. FIG. 10B shows anembodiment with two motion stops 49 _(10B). FIG. 10C shows a device thatuses two ribbon-type hinges 49 _(10C), which also inherently provide the“motion stop” functionality in two ways. As mirror 48 _(10C) deflects,hinges 50 _(10C), which may be straight in the undeflected position,take on an S shape due to torque applied by mirror 48 _(10C). As theangular deflection of mirror 48 _(10C) increases, the hinges 50 _(10C)stretch, as well as bend. Thus, the mechanical restoring force formirror 48 _(10C) increases at greater than a linear rate with referenceto angular deflection. This non-linear quality is one way by whichhinges 50 _(10C) function provide “motion stop” functionality evenwithout contacting the optically transmissive substrate 20. A second waythat “motion stop” functionality is achieved with this structure is bycontact between the mirror 48 _(10C) and hinges 50 _(10C).

[0091]FIG. 10D shows yet another embodiment of a torsion hinge device,for which the step of depositing the motion stop layer can beeliminated, since it does not make use of separately fabricated motionstops. In the embodiment of FIG. 10D, contacts 51 _(10D) are formedthrough holes in a first sacrificial layer. A ribbon hinge 50 _(10D) isformed on the first sacrificial layer. A second sacrificial layer isformed over the hinge 50 _(10D) and the first sacrificial layer andformed with a hole which exposes a central portion of hinge 50 _(10D).Contact 51 _(α) is formed through the hole and a single layer formingmirror 48 _(10D) and motion stop 49 _(10D) is deposited on top of thesecond sacrificial layer. The two sacrificial layers are then removed tofree mirror 48 _(10D) and motion stop 49 _(10D).

[0092] A third fabrication process to produce the micro-mechanicalspatial light modulator (SLM) of this invention is illustrated incross-section in FIGS. 11A-11C and FIG. 12. This process also usesmultiple silicon nitride layers to achieve a mirror-type structure witha higher aperture ratio (fraction of optically active area) than ispossible with the process outlined in FIG. 2. optically transmissivesubstrate 20 is made of a material such as quartz which can withstandsubsequent processing temperatures. In this process, the deposition ofthe light-blocking aperture layer 22, protective layer 24 has beenomitted from the process, but could be added as the first step in theprocess.

[0093] First, the optically transmissive substrate 20 is patterned andetched so that small bumps 111 are formed as contact points, as seen inFIG. 11A. Next, a 0.5 μm thick LPCVD-grown amorphous silicon sacrificiallayer 26 ₁₂ is deposited, which will eventually be removed. This isfollowed by the deposition of a 138 nm thick silicon nitride mirrorstructural support layer 28 ₁₂, which is patterned to form thesubstantially rigid mirror plate 28 ₁₂ (FIG. 11B). Next, a secondsacrificial layer 27 ₁₂ is deposited, and patterned so that hole 29 _(β)reaches down to the mirror plate 28, and so that holes 29 ₁₂ reach downto bumps 111. An approximately 40 nm thick low-stress silicon nitridehinge layer 29 ₁₂ is then grown and patterned to define the torsionhinges as seen in FIG. 11C.

[0094] Next, the sacrificial layers 26 ₁₂ and 27 ₁₂ are partiallyremoved using a xenon diflouride isotropic etch process having an etchselectivity of over 100 to 1 (a 100% SF₆ plasma process may also beused), and the entire structure is coated with a very thin layer (30 nm)of aluminum which is both highly reflective and serves to electricallyconnect the mirrors together. Finally the mirrors are fully releasedwith a second xenon diflouride etch process, completely removing thesacrificial layer 26 ₁₂. The mirrors are now ready to be joined with asemiconductor substrate containing addressing circuitry, using the samesubstrate bonding techniques described earlier in reference to FIG. 2and FIG. 3.

[0095]FIG. 12 shows an embodiment of the structure fabricated using theprocess above. Supports 51 ₁₂ are formed by the silicon nitride hingelayer deposition through holes 29 ₁₂. Hinge 50 ₁₂ is formed of hingelayer 29 ₁₂. Mirror 48 ₁₂ is the mirror plate 28 ₁₂ shown in FIG. 11B.This mirror is attached to hinge 50 ₁₂ via support 51 ₆₂ . The mirror 48₁₂ is separated from optically transmissive substrate 20 in theundeflected position due to supports 51 ₁₂.

[0096] A single square mirror is not the only possible reflectivedeflectable element 48 possible; other designs, such as a cloverleaf orgrating-like design are possible. For example, a row of skinny mirrorsall deflecting in unison can form a switchable diffraction grating. Itis also feasible that the reflective deflectable element is ametal-coated membrane. The deflectable element design could also be madeso that one part of the element moves away from the lower substrateinstead of towards it. Mirror elements can also be designed to deflectin more than one direction, i.e. have more than one controllable degreeof freedom.

[0097] If the modulator is operated so that the reflective deflectableelement touches the circuit substrate when actuated, such as would occurfor the device embodiment shown in FIG. 8E, additional structure may beadded to the circuit substrate. For example, in a mirror device,protruding bumps can be fabricated to reduce the total surface areaactually in contact. The bumps are preferably at the same electricalpotential as the mirror to avoid welding on contact. Additionally, aconducting transparent layer, such as indium tin oxide, can be depositedbefore the protective layer 24. A bias applied between the conductingtransparent layer and the mirrors will actively pull the mirrors to thetop substrate 20 and reset them to their off state.

[0098] There are many different methods to make electrical circuitrythat performs the addressing function. The DRAM, SRAM, and passiveaddressing schemes described above, as well as latch devices commonlyknown to the art, may all perform the addressing function. The circuitsubstrate may be transparent, for example, quartz. In this case,transistors may be made from polysilicon, as compared to crystallinesilicon.

[0099] In one embodiment, the aperture layer 22 may be further modifiedto comprise any binary optical pattern. In addition, other planaroptical components can be integrated into the optically transmissivesubstrate 20, at either the top surface 16 or bottom surface 14 ofoptically transmissive substrate 20. Some of the many possiblestructures include color filters composed of one or a stack of layers,micro-lenses, and color-dispersive or diffractive features. See forexample Jahns and Huang, “Planar Integration of Free-Space Opticalcomponents” Applied Optics, vol. 28, No. 9, May 1, 1989. The ability tointegrate this optical functionality into the optically transmissivesubstrate can increase achievable contrast ratio and lowers cost byreducing the cost of free-space optics at the system level. In manyembodiments of this invention, the mirror plates themselves canincorporate optical functionality beyond simple reflectivity. Forexample, the mirrors can be comprised of multiple substantiallytransparent layers to add filtering capability or to enhancereflectivity of certain wavelengths as compared to others. This isuseful, for example, as a means to balance color deficiencies of theoptical system, such as the spectrum of an illuminating lamp.

[0100] There are many fabrication process modifications which can bemade. Instead of using an epoxy to bond the two substrates together,other materials, such as metals that melt at attainable processtemperatures, or thermoplastics can be used. In any scheme, the spacerswhich hold the substrates apart can be built on either substrate. It isimportant to note that the method of deflection is also not necessarilyrestricted to electrostatic: thermal and piezo-electric actuation areamong alternate possibilities. There can also be a top to bottomsubstrate electrical connection at each pixel, where elements that makeup each pixel can be held at their own electrical potential.Chemical-mechanical polishing (CMP) can be added at several stagesduring the fabrication process, for example after the protective layerhas been deposited on top of the patterned aperture layer, or after themirror layer has been deposited, in order to make the optically activearea of the mirror as flat as possible.

[0101] Many material substitutions are possible for the micro-mechanicalelements: one possibility is the use of another type of ceramic (e.g.silicon dioxide) for the mirror, or even making the mirror completelyout of a metal (e.g. an aluminum alloy). There are also manypossibilities for the sacrificial layer material, such as silicondioxide. Silicon could also be used instead of tungsten as the gridmaterial. This would make the process more compatible with siliconnitride deposition facilities that are used for CMOS chip production.The grid and associated protective layer may also be left out entirely.Yet another combination of materials would be silicon (e.g., LPCVDpolycrystalline silicon) for the deflectable elements (e.g. mirrors),and silicon dioxide (e.g., LPCVD grown) for the sacrificial layer. Thesilicon dioxide may be etched away with hydrofluoric acid, and dryingmay be accomplished using well-known critical-point-drying techniques tofacilitate stiction-free mirror release. The spacers can also be madefrom a wide variety of materials, including various polymers, oxides, ormetals.

[0102] In summary, the SLM 10 of this invention is a device thatexhibits many desirable properties, including high resolution, highoptical efficiency, high contrast ratio or modulation depth, and highmechanical reliability. The SLM 10 has application in a wide variety ofareas, including projection display systems. Low switching voltages andthe innovative design of the SLM 10 enable standard CMOS circuitry to beused as the addressing mechanism. The deflectable elements themselvescan also be manufactured using standard processes available in siliconCMOS fabrication facilities, on a separate substrate. Both substratescan be fabricated using relatively gross features and less thanstate-of-the-art facilities. These factors make the SLM 10 easy andinexpensive to manufacture.

[0103] Although the present invention has been described above in termsof specific embodiments, it is anticipated that alterations andmodifications thereof will no doubt become apparent to those skilled inthe art. It is therefore intended that the following claims beinterpreted as covering all such alterations and modifications as fallwithin the true spirit and scope of the invention.

What is claimed is:
 1. A spatial light modulator comprising: anoptically transmissive substrate having an upper surface and a lowersurface; at least one deflectable element attached to the lower surfaceof the optically transmissive substrate; and a circuit substratepositioned below and spaced apart from the lower surface of theoptically transmissive substrate, the circuit substrate containingaddressing circuitry capable of activation of any set of the at leastone deflectable elements.
 2. The spatial light modulator of claim 1,further comprising at least one electrode connected to the addressingcircuitry, wherein each one of the at least one electrodes is positionedto selectively deflect a corresponding one or more of the at least onedeflectable elements when a bias voltage is applied between the at leastone electrode and the corresponding deflectable element.
 3. The spatiallight modulator of claim 1, wherein each of the at least one deflectableelements is reflective and comprises a metallic layer.
 4. The spatiallight modulator of claim 3, wherein each of the at least one deflectableelements further comprises a structural support layer.
 5. The spatiallight modulator of claim 1, wherein at least one of the at least onedeflectable elements further comprises: a substantially rigid platewhich is attached to the optically transmissive substrate with one ormore torsion hinges located along an edge of the plate, whereby theplate may rotate about the edge.
 6. The spatial light modulator of claim5, further comprising a means for limiting the area of contact betweeneach of the at least one deflectable element and the opticallytransmissive substrate.
 7. The spatial light modulator of claim 1,wherein the optically transmissive substrate comprises an aperturelayer, whereby light may pass only through a portion of the lowersurface of the optically transmissive substrate.
 8. The spatial lightmodulator of claim 1, wherein the optically transmissive substratecomprises fixed optical elements.
 9. The spatial light modulator ofclaim 1, wherein each of the at least one deflectable elements comprisesan electrically conductive portion and is deflectable by electrostaticforce.
 10. The spatial light modulator of claim 1, further comprising ameans for electrically connecting each of the at least one deflectiveelement to the circuit substrate.
 11. The spatial light modulator ofclaim 1, wherein each of the at least one deflectable elements issubstantially rigid and is attached to the optically transmissivesubstrate by flexible hinges.
 12. The spatial light modulator of claim1, wherein the at least one deflectable element comprises a plurality ofreflective deflectable elements, and wherein the plurality of reflectivedeflectable elements are grouped in a plurality of subsets, each subsetoriented so as to selectively direct incident light into a specificangle.
 13. The spatial light modulator of claim 1, wherein at least oneof the at least one deflectable elements is composed of a laminateincluding a metallic layer.
 14. The spatial light modulator of claim 1,wherein the circuit substrate comprises an electrode for creatingelectrostatic attraction between each of the at least one reflectivedeflectable elements and the optically transmissive substrate.
 15. Aprocess for the fabrication of a spatial light modulator, the processcomprising: depositing a sacrificial layer over an opticallytransmissive substrate; etching a hole through the sacrificial layer,whereby the hole allows for attachment of subsequent layers to thesubstrate; depositing a reflective layer over the sacrificial layer;connecting the reflective layer to the optically transmissive substratethrough the hole; patterning the reflective layer to define one or morereflective deflectable elements; removing the sacrificial layer so thatthe reflective elements are free and may deflect; forming addressingcircuitry and electrodes on a circuit substrate; and aligning andjoining the optically transmissive substrate and the circuit substrate,wherein the reflective elements may be selectively actuated by theaddressing circuitry and electrodes.
 16. The process for the fabricationof a spatial light modulator of claim 15, wherein an aperture layer isdeposited on the optically transmissive substrate before the sacrificiallayer is deposited on the optically transmissive substrate, whereby saidaperture layer allows light to pass only through a subset of thesubstrate area.
 17. A reflective spatial light modulator comprising: anarray of electrodes on a circuit substrate; an optically transmissivesubstrate comprising a lower side, the optically transmissive substratebeing positioned above and spaced apart from the circuit substrate, theoptically transmissive substrate having an array of conductingreflective deflectable elements corresponding to the array of electrodesand attached to the lower side.
 18. The spatial light modulator of claim17, wherein: the reflective deflectable elements are electricallyconnected in rows; the electrodes are electrically connected in columnsthat cross the rows at pixel locations; whereby individual pixels may beturned on and off by selectively applying appropriate row and columnbiases and creating electrostatic attraction.
 19. The reflective spatiallight modulator structure of claim 17, wherein the opticallytransmissive substrate contains an aperture layer, wherein said aperturelayer allows light to pass only through a subset of the substrate area.20. The spatial light modulator of claim 1, wherein each of the at leastone deflectable element comprises a mirror having an angle with respectto the optically transmissive substrate, wherein the angle can be variedcontinuously by actuating a corresponding electrode in the addressingcircuitry.
 21. The spatial light modulator of claim 1, wherein each ofthe at least one deflectable elements further comprises a mirror stoprigidly connected to the mirror such that when the angle increases, afree end of the mirror stop moves closer to the optically transmissivesubstrate.
 22. The spatial light modulator of claim 21, wherein themirror stop is structured such that a free end of the mirror is separatefrom the circuit substrate when the free end of the mirror stop is incontact with the optically transmissive substrate.
 23. The spatial lightmodulator of claim 21, wherein each of said at least one deflectableelements is connected to the circuit substrate by a hinge such that eachof the at least one deflectable elements is free to rotate about acorresponding hinge, wherein the mirror stop is connected to the hingeopposite the mirror.
 24. The spatial light modulator of claim 21,wherein the mirror stop comprises a sharp contact tip configured tocontact the optically transmissive substrate when the angle is at amaximum value.
 25. The spatial light modulator of claim 21, wherein themirror stop is coplanar with the mirror.
 26. The spatial light modulatorof claim 21, wherein the mirror stop and the optically transmissivesubstrate are electrically connected.
 27. The spatial light modulator ofclaim 1, wherein at least one of the at least one deflectable elementscomprises: a mirror plate; and a hinge connecting the mirror plate tothe optically transmissive substrate, the hinge structured such thatwhen a force is applied to the mirror plate, bending occurs in thehinge, and, as a result, an angle between the mirror plate and theoptically transmissive substrate changes.
 28. The spatial lightmodulator of claim 27, where the hinge is disposed along an edge of themirror plate.
 29. The spatial light modulator of claim 27, wherein thehinge is relatively elastic compared to the mirror plate.
 30. Thespatial light modulator of claim 29, wherein the hinge material has asmaller modulus of elasticity than the mirror material.
 31. The spatiallight modulator of claim 30, wherein the hinge is thinner than themirror plate.
 32. The spatial light modulator of claim 27, wherein thehinge lies in a different plane from the mirror plate.
 33. The spatiallight modulator of claim 32, wherein the hinge is disposed between themirror plate and the optically transmissive substrate.
 34. The spatiallight modulator of claim 33, wherein the hinge is composed ofsubstantially transparent material.
 35. The spatial light modulator ofclaim 32, wherein the mirror plate is disposed between the hinge and theoptically transmissive substrate.
 36. The spatial light modulator ofclaim 1, wherein at least one of the at least one deflectable elements,when activated, deflects towards the addressing circuitry.
 37. Thespatial light modulator of claim 36, wherein a voltage of saidaddressing circuitry required to cause the at least one deflectableelements to snap towards the addressing circuitry is greater than avoltage of said addressing circuitry at which the deflectable element isreleased from the addressing circuitry.
 38. The spatial light modulatorof claim 1, wherein the spatial light modulator comprises a portion of atwo-dimensional array of spatial light modulators.
 39. The spatial lightmodulator of claim 1, wherein the addressing circuitry comprises amemory array.
 40. The spatial light modulator of claim 40, wherein thememory array comprises a DRAM memory array.
 41. The spatial lightmodulator of claim 40, wherein the memory array comprises an SRAM memoryarray.
 42. The spatial light modulator of claim 1, wherein the circuitsubstrate comprises a portion of a silicon die.
 43. The spatial lightmodulator of claim 1, wherein the optically transmissive substratecomprises quartz.
 44. The spatial light modulator of claim 1, furthercomprising: a light source configured to propagate electromagneticradiation onto at least one of the reflective deflectable element; andimage optics configured to receive at least a portion of the reflectedelectromagnetic radiation.
 45. The spatial light modulator of claim 1,wherein the at least one deflectable element comprises: an opticallytransparent support layer; and a reflective layer.
 46. The spatial lightmodulator of claim 45, wherein the optically transparent support layercomprises a silicon nitride layer.
 47. The spatial light modulator ofclaim 45, wherein the reflective layer comprises an aluminum layer. 48.The spatial light modulator of claim 1, further comprising a voltagesource configured to assert a voltage bias to one or more subsets of theat least one reflective deflectable elements.
 49. The process of claim15, wherein removing the sacrificial layer comprises etching thesacrificial layer with a XeF₂ gas phase etch to release the reflectivedeflectable element except at a hinge connecting the reflectivedeflectable element with the optically transmissive substrate.
 50. Theprocess of claim 15, wherein depositing a reflective layer comprises:depositing a first dielectric layer of a first index of refraction; anddepositing a second dielectric layer of a second index of refraction,different from the first index of refraction.
 51. The process of claim15, wherein patterning the reflective layer comprises: depositing anoptically transmissive layer; patterning the optically transmissivelayer to define a mirror stop, the mirror stop having a free end; anddepositing a conductive material on the mirror stop such that theconductive material does not contact the optically transmissivesubstrate when the mirror stop contacts the optically transmissivesubstrate.
 52. The process of claim 51, wherein depositing a conductivematerial comprises: depositing the conductive material at an angletowards the free end of the mirror stop.
 53. A process for operating aspatial light modulator comprising: asserting a bias voltage betweenaddressing circuitry and a reflective deflectable element, theaddressing circuitry being contained in a circuit substrate, thereflective deflectable element being attached to a lower surface of anoptically transmissive substrate, the circuit substrate disposed below,but separated from, the optically transmissive substrate.
 54. Theprocess of claim 53, wherein asserting a bias voltage comprises:asserting a negative voltage to the reflective deflectable element; andasserting a positive voltage from 0 to 5 volts to the addressingcircuitry.
 55. The spatial light modulator of claim 5, furthercomprising a means for limiting the area of contact between each of theat least one deflectable element and the circuit substrate.